DONETSK, Ukraine — When two armed men on Thursday afternoon barged into the small yellow schoolroom that has served as Inna Mashenskaya’s makeshift election office, she stood up straight and began to argue. But when one of them, wearing sunglasses, patted the knife that was hanging from his belt, she stopped talking and surrendered control of her polling center.

“That is an argument I cannot win,” she said.

As Ukraine hurtles toward a presidential election on Sunday, the first national vote since an uprising toppled the elected government this year, Ukraine’s troubled east has emerged as the most serious risk to the vote and the country’s future.

A new burst of violence, some of the worst in months, left at least 16 Ukrainian soldiers dead on Thursday, giving new life to what had appeared to be a waning conflict just three days before the critical vote, which Western governments hope will lift the country out of violent upheaval into the relative safety of politics.

Yet the ambush of a Ukrainian military checkpoint was a major setback for the authorities in Kiev, and it suggested a renewed push by anti-Kiev fighters after weeks of growing disaffection with them among ordinary people here and increasingly rancorous divisions among their leaders.

The attack complicates an already troubled situation. Local election observers estimate that only about a tenth of polling stations will be able to open on Election Day in the Donetsk region — the country’s largest voting district, with about 10 percent of the population — undermining the credibility of the vote. Western observers have flooded into Ukraine, but so far the vast majority of them have chosen to avoid the eastern areas that are controlled by pro-Russian fighters.

Fearing kidnappings of foreigners, the local authorities are not objecting.

“I recommended that they not come,” said Valeriy Zhaldak, an aide to the governor of Donetsk, referring to foreign observers. “We’d be happy to see them, but it would end up as our headache.”

The south and the east make up nearly half of the nation’s population — a crucial part of Ukraine’s collective voice. But people here are deeply conflicted about a country that most still want to be part of but in some ways no longer recognize as their own.

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A deadly ambush of government soldiers in eastern Ukraine added to fears that voting on Sunday in that part of the country would be compromised, casting doubt on the results’ credibility.CreditIvan Sekretarev/Associated Press

“You have this whole belt of discontent in the south and the east that doesn’t accept the legitimacy of Kiev, but doesn’t have an alternative option,” said Keith Darden, a political scientist at American University in Washington. “That’s the main story, and it’s really not clear how it ends.”

The deposed leader, Victor F. Yanukovych, was from the east, and few here have any affection for him. Several of the current candidates come from the east and south, but they are strongly associated with Kiev and have little appeal to voters here.

“There’s no one to vote for,” said a 21-year-old rugby coach named Daniel. “No one liked Yanukovych — O.K. — but he was elected and legal. Why couldn’t they have just waited,” he asked, for the next election, which had been scheduled for March 2015.

The ambush of the military checkpoint happened just before dawn on a highway near the town of Blahodatne, about an hour’s drive southwest of Donetsk. The military said that militants attacked the checkpoint with automatic weapons and grenades before fleeing in several vehicles, including armored vans.

A spokesman for the Donetsk regional governor, citing the health department, said that 16 Ukrainian soldiers had been killed. Aleksandr Bezugly, the head doctor for the hospital in nearby Volnovakha, said more than 20 had been hospitalized.

A driver for the hospital said three bodies were badly burned around the neck and head. A burned armored personnel carrier lay on its side at the checkpoint, said the hospital driver, who identified himself only as Nikolai.

“Probably they were sleeping in the armored vehicles when they attacked,” he said, eating a sandwich in the concrete garage at the hospital. “And they burned.”

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A Ukrainian soldier stood next to a destroyed truck on Thursday at the scene of fighting near the village of Blahodatne, eastern Ukraine.CreditIvan Sekretarev/Associated Press

Residents in Olginka, a village of houses and farms, said they had heard shooting just before dawn, but could not tell who had attacked the checkpoint. The checkpoint had appeared on the country road earlier this week, and several women in the village said that they had brought water and salo, a form of lard popular in Ukraine, to the soldiers.

Klavdia Kulbatskaya, a spokeswoman for the Donetsk People’s Republic, said the republic was not involved in the attack, and the identity of the attackers remained in dispute.

The new fighting broke out as, for the first time, Anders Fogh Rasmussen, the secretary general of NATO, offered a cautious and qualified confirmation that Moscow may be preparing to pull back some forces from areas near the border with Ukraine.

But the authorities in Kiev continued to blame Moscow for the unrest. Ukraine’s acting prime minister, Arseniy P. Yatsenyuk, accused Russia of escalating the conflict and of trying to disrupt Sunday’s election, according to a statement on Facebook.

Election observers in Donetsk offered grim assessments. Sergey Tkachenko, head of the Donetsk chapter of the Committee of Voters of Ukraine, a nonprofit group that monitors elections, said about eight election commissions had been seized by militants in the Donetsk region, and work at countless polling centers had not begun, out of fear they would be threatened, including some in Mariupol, a southern city where worker patrols have sharply reduced the clout of the separatists.

“Chaos will be up to here,” Mr. Tkachenko said, drawing a line across his throat with his index finger.

Ms. Mashenskaya, the poll worker, had worked for weeks to keep her polling center open. She had received neither pay nor voting materials after an earlier incident involving men in masks shut her headquarters. So she bought her official registry book at an office supply store, brought paper and pencils from home, and scrounged for fabric to drape over voting booths.

Her enthusiasm came less from a love of the democratic process and more from a fear of what her country might become without it.

“We’re like a suitcase without a handle — there’s something inside but no way to bring it anywhere,” said Ms. Mashenskaya, who is a psychologist. “I know we will elect someone who will give us a headache. But if there’s no election, we’re stuck in this dead end.”

A version of this article appears in print on , on Page A5 of the New York edition with the headline: Violence and Doubts About Credibility as a Troubled Ukraine Election Nears. Order Reprints | Today’s Paper | Subscribe